Field of the Invention
This invention relates to low-temperature soldering processes. This invention also relates to wax-flux compositions useful in such low-temperature soldering processes.
A commercially popular labor saving process for assembling electronic components is to install these components on a printed circuit board. After most, if not all, of the components are installed on the circuit board, the components are soldered into place by a process known as wave soldering.
One difficulty with the wave soldering process is that the components have leads which extend 1-2 inches beyond the surface of the printed circuit board. This long length requires a very high standing wave of solder. The standing wave can be forced high enough to solder these long lead components but this has disadvantages. Among them, a large amount of solder is lost in coating all these long leads and large globs of solder form between groups of leads which extend below the board. However, by soldering the printed circuit board prior to lead trimming, the components are fixed in place and can be trimmed by mechanical means rather than being hand trimmed, although hand-trimming, of course, is quite feasible.
The disadvantage of trimming a circuit board after it has been soldered is that critical applications, such as military or space applications require the circuit boards be resoldered subsequent to the trimming. This, of course, greatly increases the expense to the party doing the soldering as well as increases the risk of damage to the components.
An alternative to soldering the long leads is to hand trim them and bend the stubs over against the board before soldering. The bent-over stubs hold the components in place during the soldering operation. Advantages of this method are that only one soldering operation is required and only a low wave of solder need be maintained. The disadvantage is the large amount of hand labor required by this method.
What is needed is an inexpensive method which does not involve solder to fix the components in place on the circuit board while the leads are being trimmed and which does not interfere with the subsequent soldering operation.
Wave soldering also uses a flux to clean the printed circuit board and prepare both the component leads and the printed circuit board foil to receive the solder. In a typical commercial installation, a liquid flux is maintained in a bath which contains fritted aspirators. Aspirating the liquid results in a foam head on the flux tank. The amount of aspiration is controlled so that the foam head just touches the printed circuit boards conveyed above it. A light film of the liquid flux is then deposited on the printed circuit board which subsequently passes over a heater. This preheats the circuit board. The printed circuit board immediately thereafter passes over the standing wave of solder and the soldering process is accomplished.
One disadvantage of using this method of applying the soldering flux to the parts which are to be soldered is that extra equipment is needed. In addition, the flux must be aspirated very carefully and constant attention must be paid to the machinery to see that the flux head remains at the right height. If the flux head is too low, obviously the printed circuit boards will not be properly treated and a poor soldering job will result. If the flux head is too high, the entire circuit board including the components may be coated with flux which requires additional cleaning as well as wastes soldering flux. What is needed is a convenient way to apply the soldering flux to the printed circuit board which does not require special equipment and special attention.
Certain sulfonic acids, notably the alkylaryl sulfonic acids have been suggested as suitable fluxing agents for wax-flux compositions. These acids are conveniently prepared by oleum sulfonation. However, this process yields sulfonic acids containing a certain amount (often up to 2% weight or more) of free sulfuric acid. Sulfonic acid fluxes which contain free sulfuric acid tend to stain phenolic printed circuit boards brown even at room temperature, but particularly so at elevated (e.g., soldering) temperatures.
The present invention not only solves the problem of how to firmly fix the components to the printed circuit board for the lead-trimming operation, but also the problem of how to conveniently apply soldering flux to the printed circuit board all in a single-step operation. The present invention also solves the problem of staining of phenolic printed circuit boards caused by the presence of free sulfuric acid in the sulfonic acids.